Electromechanical converter comprising at least one piezoelectric element

ABSTRACT

An electromechanical transducer with at least one piezoelectric element with a first surface on which a first electrode is applied, and at least one contact flag, through which the piezoelectric element is electrically connected. The electrical connection of the piezoelectric element is of very high contact reliability, in that the contact flag directly contacts the first electrode and exhibits at least one depression, into which an adhesive is placed, by which the contact flag is connected with the electrode.

FIELD OF THE INVENTION

The invention relates to an electromechanical transducer with at leastone piezoelectric element.

BACKGROUND OF THE INVENTION

Today there are many devices, especially measuring instruments andsensors, in which piezoelectric elements are used as electromechanicaltransducers.

There are e.g. fill level limit switches commercially available, whichare used for limit level detection, overfill prevention or protectionagainst pumps running dry. These have e.g. a mechanical oscillationstructure extending into a container. The structure is caused tooscillate by means of a piezoelectric element or by means of a pluralityof piezoelectric elements arranged in a stack. The resonance frequencyand/or the amplitude of an excited oscillation is registered, and fromthis it is determined whether the oscillation structure is oscillatingfreely or, instead, covered with a fill material.

The piezoelectric elements are usually provided with electrodes and areconnected by means of electrical contact flags or lugs. The contactflags are connected with an electronic circuit by way of electricalconductors, through which the piezoelectric elements are either excitedto oscillate, or their oscillation-dependent signals are registered.

Examples of electromechanical transducers with one or more piezoelectricelements are described in EP-A 875 742, EP-A 875 741, EP-A 875 740, EP-A875 739 and European Patent Application 10028319.5 filed 7 Jun. 2000.

Presently, the contact flags are adhesively attached to the electrodes.Adhesives are always applied onto a flat area of the contact surfaces ofthe electrodes, or of the contact flags, as the case may be. Thethickness of the adhesive layer depends in such practice on the amountof adhesive applied, the contact pressure under which the parts to bebonded are brought together, and the, in part, alsotemperature-dependent viscosity of the adhesive before it sets. Thefinite thickness of the adhesive layer means that there will be a gapbetween the electrode and the contact flag. Such a gap occurs naturallyalso in the alternative practice where the contact flag is provided withsolder dots and the electrode is soldered on.

With time, the adhesive experiences aging effects, which can affect thecontact reliability of the connection.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an electromechanicaltransducer having at least one piezoelectric element, characterized byan electrical connection of the piezoelectric element with a very highcontact reliability.

To accomplish this object, the invention comprises an electromechanicaltransducer with

-   -   at least one piezoelectric element with a first surface on which        a first electrode is applied, and    -   at least one contact flag,        -   through which the piezoelectric element is electrically            connected,        -   which directly contacts the first electrode, and        -   which exhibits at least one depression,        -   into which an adhesive is placed, by which the contact flag            is connected with the electrode.

In one development of the invention, the adhesive is electricallyconductive.

In one development of the invention, the adhesive is an adhesive that isusable for the securement of SMD-components.

In one development of the invention, the depression is etched into thecontact flag.

In one development of the invention, the piezoelectric element has onthe first surface at least one more electrode, and, for each electrode,a contact flag is provided, which electrically connects its associatedelectrode.

In one development of the invention, the contact flags are connectionflags extending from a circuit board, particularly a printed circuitboard, and the connection flags become conductor paths in the circuitboard.

In one development of the invention, the circuit board is a flexiblecircuit board, which has a plurality of contact flags, wherein, on eachcontact flag, a piezoelectric element is arranged, and the piezoelectricelements are arranged in a stack.

Additionally, the invention includes a method for producing anabove-mentioned electromechanical transducer, wherein the depressions inthe contact flags are etched in, adhesive is placed in the depressions,and the piezoelectric element is set onto the contact flags by machine.

The invention and further advantages will now be explained in moredetail on the basis of the figures of the drawing, where three examplesof embodiments are presented; equal elements are provided in the figureswith the same reference characters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a piezoelectric element with a contact flagconnected therewith;

FIG. 2 shows a plan view of the side of the contact flag facing thepiezoelectric element;

FIG. 3 shows a view of a piezoelectric element where two electrodes arearranged on a first surface;

FIG. 4 shows a flexible circuit board having two contact flags;

FIG. 5 shows a flexible circuit board with six contact flags; and

FIG. 6 shows piezoelectric elements arranged in a stack and connected oncontact flags.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows schematically an electromechanical transducer. Thetransducer includes a piezoelectric element 1. The piezoelectric element1 is in the form of a circular disk in the illustrated embodiment andexhibits a first surface, on which a first electrode 3 is located. Thefirst electrode 3 is e.g. a thin silver layer, which is applied on thepiezoelectric element 1, e.g. by sputtering. A contact flag 5 isprovided, over which the piezoelectric element 1 is electricallyconnected.

FIG. 2 shows a plan view of the side of the contact flag facing thepiezoelectric element. The contact flag 5 has a circular, disk-shapedsection 6, the surface of which directly contacts the first electrode 3.The contact flag 5 contains at least one depression 7, and an adhesive 9is placed in each depression. The embodiment of FIG. 1 has twodepressions. The adhesive 9 connects the contact flag 5 with theelectrode 3.

Especially suited to serve as adhesive 9 are e.g. an electricallyconductive adhesive, or an adhesive of the type used for securing SMDcomponents.

In the illustrated embodiment, contact flag 5 is an integral part of aflexible circuit board 11. Flexible circuit boards are available e.g.from the firm Schoeller Elektronik under the trademark Polyflex. Theyare made e.g. of a thin copper sheet, which is first processed by thefirm Schoeller Elektronik in an etching process according to a desiredconductor path configuration. Then, both sides of the resultingconfiguration are laminated with a thick polyimide covering film.

The embodiment illustrated in FIG. 2 uses a flexible circuit board 11,in which the contact flag 5 is a connection flag of sheet materialextending from the flexible circuit board 11. The connection flag is anintegral part of the flexible circuit board 11. It is e.g. anappropriately formed segment of the copper sheet, without the coveringfilm. The depressions 7 are preferably etched into the contact flag 5.The depressions have e.g. a depth of 3 μm.

Preferably, large amounts of flexible circuit boards are processed bymachine batchwise. An etching mask is used to guide the etching of thedepressions 7 on all circuit boards of the batch. Then, the adhesive 9is applied by machine from a dispenser or by screen printing. In a lastprocessing step, the piezoelectric elements 1 are positioned and pressedin place. If an adhesive 9 is used that shrinks during hardening, thenthe two components, contact flag and piezoelectric element withelectrode, are additionally pulled together by the bond provided byadhesive 9.

Since the adhesive remains completely in the depressions 7, a quasigap-free electric coupling of the contact flag 5 onto the electrode 3 isobtained. The separation between the contact flag 5 and the electrode 3is essentially determined by the manufacturing tolerances in theproduction of electrode 3 and contact flag 5. The separation liesclearly below 10 μm. At these small separations, there is, in thequality of the electrical connection, no longer any difference between acapacitive and a conductive connection.

Instead of a single electrode arranged on the piezoelectric element,there can, of course, be two or more electrodes provided, each of whichis connected in the above-described manner to a contact flag dedicatedto its particular electrode.

FIG. 3 shows a view of a first surface of a piezoelectric element 13,where two electrodes 15, 17 are arranged on the first surface. Theelectrodes 15, 17 are of semicircular shape and can e.g. serve to excitethe two halves of a circular-disk-shaped, homogeneously polarized,piezoelectric element to execute oscillations of opposite phase. Otherapplications of piezoelectric elements with at least two electrodes aredescribed e.g. in EP-A 875 740 and EP-A 875 739.

FIG. 4 shows a view of a flexible circuit board 19 usable in connectionwith the piezoelectric element 13 illustrated in FIG. 3. The circuitboard 19 has contact flags 21, 23, one for each electrode 15, 17, toelectrically connect the electrodes 15, 17. In the embodiment of FIGS. 3and 4, each contact flag 21, 23 has a depression 25, 27 for receivingthe adhesive. The bonding and the electrical connection of theelectrodes 15, 17 proceeds analogously to the bonding and the electricalconnection as it is explained above in the description of FIGS. 1 and 2.

Also the embodiment illustrated in FIG. 4 uses a flexible circuit board,in which the contact flags 21, 23 are connection flags of sheet materialextending from the flexible circuit board 19. The connection flags areintegral parts of the flexible circuit board 11. They are e.g.appropriately formed segments of the copper sheet. The segments in thisembodiment are provided with a covering film 29 on the side opposite tothat facing the piezoelectric element 13. The sides of the contact flags21, 23 facing the piezoelectric element 13 are bare. In the circuitboard 19, the contact flags 21, 23 become conductor paths extending inthe circuit board 19. The depressions 25, 27 are, in this case also,etched into the contact flags 21, 23 to a depth of about 3 μm.

FIGS. 5 and 6 illustrate another embodiment of an electromechanicaltransducer of the invention. FIG. 5 shows a view of a flexible circuitboard, which has a number of contact flags 31 extending from a flexiblecircuit board, as described previously in connection with FIGS. 1–4.FIG. 6 shows the circuit board of FIG. 5, where now a piezoelectricelement 33 is arranged on each contact flag 31, and the piezoelectricelements 33 are arranged in a stack.

The flexible circuit board has a branched section 37. Six branches arepresent in this embodiment. At the root of each branch, a connectionflag of sheet material extends outwards to serve as a contact flag 31.Each connection flag is composed of a narrow path extending from theroot out to a circular-disk-shaped electrode area. Provided in eachelectrode area are three depressions 39 which receive adhesiveanalogously to the previous embodiments.

Each two branches are separated from one another by a distance equal tothe thickness of the piezoelectric element 33 between them.

In the mounting of the piezoelectric elements 33, an adhesive, e.g. aconductive adhesive or an SMD-adhesive, is placed in the depressions 39and then the piezoelectric element 33 is applied. Each piezoelectricelement 33 with an electrode 40 (hatched dark in FIG. 6) comes to lie inflush contact with an associated contact flag 31.

Following this, the flexible circuit board is deformed to arrange thecontact flags 31 to be parallel to one another and mutuallysuperimposed. In the embodiment illustrated here, as this is done, thecontact flags 31 become aligned perpendicularly to the section 37, andsection 37 rolls up, beginning from the end having the lowest branch.The piezoelectric elements 33 become stacked one on top of another. Thethus pre-formed stack is then placed between two hemispherical platens41 and pressed, to assure a good electrical contact between the contactflags 31 and the piezoelectric elements 33.

The form of flexible circuit board illustrated here is described in theEuropean Patent Application 10028319.5 filed 7 Jun. 2000. Other variantsof piezoelectric elements arranged in a stack are disclosed therein, forwhich the invention described here is analogously applicable.

The contact flags 31 become conductor paths within the circuit board.These paths permit the individual piezoelectric elements to beelectrically addressable.

Depending on the circuitry, the separate piezoelectric elements 33 areconnected electrically in series or parallel. Also, a plurality ofpiezoelectric elements 33 can be connected in series to form atransmitter and others in series to form a receiver.

The circuit board has a narrow extension 43 extending perpendicularly tothe section 37. A plug 45 is provided on the end of the extension 43.Extension 43 contains all conductors of the circuit board which requirea connection external to the circuit board.

Besides the piezoelectric elements 33, the circuit board carrieselectronic components, preferably SMD-components 35. These are e.g.current and voltage controlling elements or even components of a signalreceiving and/or signal processing circuit. SMD-components 35 can bemounted together with the piezoelectric elements 33 in one process step.

In all described embodiments, the depressions 7, 25, 27, 39 are etchedinto the contact flags 5, 29, 31 in a first process step. Then, theadhesive is placed in the depressions 7, 25, 27, 39, preferably bymachine, e.g. with a dispenser or by screen printing, following whichthe piezoelectric elements 1, 13, 33 are placed on the contact flags 7,25, 27, 33, preferably also by machine. In doing this, the piezoelectricelements 1, 13, 33 are handled preferably as SMD-components and placedon the circuit boards together with possibly required SMD-components 35by means of an automatic populating machine.

Placement of the adhesive 9 in the depressions 7, 25, 27, 39 assures avery good and constant electrical connection of the piezoelectricelements 1, 13, 33. At the same time, a fully automatable manufacturingprocess is possible.

1. An electromechanical transducer, comprising: a first electrode; atleast one piezoelectric element with a first surface on which said firstelectrode is applied; and at least one contact flag, wherein: said atleast one piezoelectric element is electrically connected through saidat least one contact flag, said at least one contact flag directlycontacts said first electrode; and said at least one contact flag has atleast one depression, into which an adhesive is placed, by which saidcontact flag is connected with said first electrode.
 2. Theelectromechanical transducer as defined in claim 1, wherein: saidadhesive is electrically conductive.
 3. The electromechanical transduceras defined in claim 1, wherein: said adhesive is an adhesive that isusable for the securement of SMD-components.
 4. The electromechanicaltransducer as defined in claim 1, wherein: said at least one depressionis etched into said contact flag.
 5. The electromechanical transducer asdefined in claim 1, wherein: said at least one piezoelectric element hasat least one more electrode on said first surface; and for eachelectrode, a contact flag is provided, which electrically connects itsassociated electrode.
 6. The electromechanical transducer as defined inclaim 1, wherein: said contact flags are connection flags extending froma circuit board; and in the circuit board, said connection flags becomeconductor paths in the circuit board.
 7. The electromechanicaltransducer as defined in claim 6, wherein: the circuit board is aflexible circuit board, which has a plurality of contact flags, on eachcontact flag, a piezoelectric element is arranged; and the piezoelectricelements are arranged in a stack.